This invention relates to electromagnetic surveying, and is applicable, for example, to surveying buried elongate conductors such as pipes and cables, for monitoring or location.
It is known to survey a buried elongate conductor by monitoring an a.c. signal current in the conductor. The a.c. signal may be deliberately applied to the conductor for this purpose (directly or inductively) or in some cases, use is made of currents of other origins, such as the mains current in a power cable or naturally induced mains or broader spectrum frequencies in other conductors.
Patent specifications GB No. 15 77 742 and EP No. 0 036 257 show such devices in which there are two or three transducer coils for monitoring the current in the conductor, with their axes arranged in parallel, horizontal, and vertically spaced apart. Naturally, the signals induced in the coil furthest from the conductor are smaller than those in the nearest coil, and this fact is used when processing the signals to give a value indicative of the depth of the conductor below the device. This value is then used as a correction factor, enabling a value indicative of the absolute value of the signal current in the conductor at any particular location to be displayed. If the conductor is in sound condition the signal current will decrease gradually the further one is from the signal source. However, if there is a marked change in the signal current at any location, this is indicative of current leakage caused by a deterioration in the conductor at that location, usually due to deterioration of the protective, insulating wrapping around the conductor. If such deterioration goes unchecked, corrosion of the conductor is likely to occur, and in the case of a pipeline, for example, leakage of the contents of the pipe which may be the result of such corrosion will show up as an even more marked change in the signal current detected. Thus, such monitoring devices can be used both for fault finding and for preventative surveying of buried conductors, without the need for digging.
However, one problem which is common to these known devices is that even once the operator is sure that he is directly over the conductor (so that the depth measurement is reliable), he has to give the device the correct azimuth orientation so as to obtain an accurate reading of signal current intensity. That is, he must physically rotate the coil arrangement about a vertical axis until a maximum signal intensity is obtained. This will occur when the horizontal coil axes are perpendicular to the direction in which the elongate conductor runs. Thus, when taking an accurate reading, the operator must first of all orientate his apparatus by trial and error until a maximum signal is achieved. This is a skilled and time-consuming operation.